Rapid increase of cellular service subscribers and wireless applications has stimulated research efforts in developing wireless communication systems that support reliable high rate transmissions over wireless channels. A major challenge in designing high-performance, high-rate systems is the mitigation of fading propagation effects within the prescribed bandwidth and power limitations. In order to mitigate deleterious effects fading has on system performance, transmitters and receivers that exploit available diversity have been developed. To this end, multi-input multi-output (MIMO) wireless links are particularly attractive when compared to single-input single-output wireless links. Existing MIMO designs utilizing multiple (Nt) transmit antennas and multiple (Nr) receive antennas aim primarily at achieving either high performance or high rate.
MIMO systems that achieve high performance by utilizing the available space-diversity. Space-time (ST) orthogonal designs (OD), linear constellation preceding (LCP) ST codes, and ST trellis codes are examples of performance driven designs. ST-OD codes can achieve full diversity (FD), i.e. the product of transmit and receive antennas, with linear decoding complexity. ST-OD systems with (Nt, Nr)=(2, 1) antennas can achieve transmission rates up to one symbol per channel use (pcu). However, relative to full rate (FR) MIMO designs capable of Nt symbols pcu, other ST-OD codes incur significant rate loss. For example, ST-OD systems with Nt>2 have transmissions rates less than 0.75 symbols pcu. ST-TC schemes can offer better transmission rates, but are complex to decode and lack closed-form construction while their design complexity increases exponentially with Nt. In addition, high rate high performance ST-TC often require long size block resulting in long decoding delays.
MIMO systems designed to achieve high rate utilize the capacity of MIMO fading channels. Bell Laboratories layered space time architecture (BLAST)-type architectures and linear dispersion (LD) codes are examples of rate driven designs. LD designs offer no diversity guarantees. LD designs including diversity constraints are imposed require a search over a high dimensional space that becomes prohibitively complex as Nt and the constellation size increase. However, layered ST multiplexers have complementary strengths and limitations. For example vertical-BLAST (V-BLAST) offers FR, i.e. Nt symbols pcu, but relies on single-input single-output (SISO) error control coding per layer to offer performance guarantees. On the other hand, Diagonal-BLAST (D-BLAST) systems utilize space diversity but have rate improvements that come at the price of increasing decoding delays. Nevertheless, V-BLAST and D-BLAST can afford reasonable complexity decoding and facilitate SISO codes in MIMO systems. However, the rate efficiency of V-BLAST and D-BLAST schemes is offset by the bandwidth consuming SISO codes required to gain diversity. In other words, both high performance and high rate ST codes do not take full advantage of the diversity and capacity provided by MIMO channels. Furthermore, conventional schemes are not flexible to strike desirable tradeoffs among performance, rate, and complexity.